This is an R21 proposal to identify brain regions that participate in the neuroadaptive responses underlying tolerance to nitrous oxide (N2O) hypothermia. Addictive drugs perturb variables that are subject to homeostatic control. Drug effects are thereby countered by adaptive biological responses. Many addiction researchers believe that these adaptive changes give rise to central features of the addiction phenotype, including drug tolerance, dependence and withdrawal. During an initial drug administration, reflexive adaptive responses can oppose a drug effect, causing acute (intrasessional) tolerance. With repeated drug exposures, the magnitude and timing of adaptive responses improve such that the drug has little observable effect (i.e., chronic tolerance develops). We propose to use c-fos immunostaining to identify brain areas that participate in neuroadaptive responses to the pharmacologically-active gas, N2O. The proposed experiments exploit the principle that adaptive response mechanisms are activated only if the regulated variable is perturbed by the drug. As with ethanol, N2O has a potent hypothermic effect at typical room temperatures, and tolerance develops to this, both acutely and chronically. However, preventing ethanol-induced hypothermia by elevating ambient temperature prevents development of chronic tolerance despite repeated drug administrations because the adaptive responses are not recruited. Accordingly, we will manipulate ambient temperature to either provide (21 degrees C) or deny (30 degrees C) rats the hypothermic state that typically occurs with 60% nitrous oxide. Two experiments are proposed to investigate the adaptive responses that develop acutely during an initial nitrous oxide exposure, as well as chronically over repeated exposures. Core temperature, and c-fos gene expression, a widely-used measure of neuronal activity, are the primary dependent measures. This work will be the first to identify brain areas activated by a drug-induced hypothermic stimulus and its adaptive consequences. These studies have theoretical importance for understanding and treating drug addiction.